MODE-1818 Repository configuration can now dictate garbage collection options.

The garbage collection options can now be specified in the repository configuration. Each repository registers its own garbage collection processes with a named thread pool, and these processes are stopped when the repository is shut down.

Added the garbage collection configuration attributes to ModeShape's XSD schema and subsystem, and added a unit test that verifies the attributes are read correctly.

This should be merged into the 'master' branch. If this is too complex to be merged into '3.1.x', then we can create a separate pull-request for '3.1.x' that just just adjusts the hard-coded frequency of the binary garbage collection.